The present invention relates to a dispersion shifted optical fiber and may be used singly or in combination with a dispersion compensating optical fiber and the like in a transmission path in an optical communication system that uses one type or two or more types of optical fibers, and may also be used for transmitting high power signal light and for performing wavelength multiplex transmissions in the above types of optical communication system.
The wavelength with the lowest loss in quartz based optical fibers is in the vicinity of 1.55 xcexcm and, conventionally, this wavelength band has been used for long distance transmissions. As the transmission path (i.e. optical fiber) in this case, generally, a dispersion shifted fiber (DSF) is used that has been designed such that the absolute value of the chromatic dispersion values in the 1.55 xcexcm wavelength band is small.
Moreover, optical communication systems that perform division multiplexing on the signal wavelength (WDM) and make use of high power signals utilizing light amplifiers such as EDFA (erbium doped optical fiber amplifiers) have appeared in recent years in response to the demands for even greater volumes in optical communication. In this case, because of the high intensity of the optical power transmitted through the fiber, the deteriorations in transmission due to non-linear optical effects cannot be ignored.
Furthermore, in conventional optical communication systems, the vicinity of 1530 to 1570 nm wavelength band has been used, however, recently, investigations have been progressing into the further expansion of the transmission volume in wavelength multiplex transmission systems. For example, devices in the range of 1570 to 1625 nm have been developed and results of investigations of the 1490 to 1530 nm wavelength band have also been made public. Currently, these wavelength bands that are in actual use or are under investigation are generally known by the following terms. Namely, the 490 to 1530 nm band is known as the S-band; the 1530 to 1570 nm band is known as the C-band; and the 1570 to 1630 nm band is known as the L-band. In actual practice, the used wavelength band (operating wavelength band) in an optical communication system may be appropriately selected from the range of 1490 to 1625 nm.
The non-linear optical effect of the transmission path is evaluated using a non-linear constant represented by n2/Aeff. n2 is the non-linear refractive index of an optical fiber and Aeff is the effective core area (effective area) of the optical fiber.
In order to reduce the non-linear effect, it is necessary to reduce the non-linear constant n2/Aeff. Because n2 does not change greatly once the material has been decided, conventionally, attempts have generally been made to reduce the non-linear constant by enlarging the Aeff.
The present inventors have proposed in, for example, Japanese Unexamined Patent Application, First-Publication (JP-A) Nos. 10-62640, 10-293225, and the like a dispersion shifted fiber with a far larger Aeff than a conventional dispersion shifted fiber as a dispersion shifted fiber suitable for long distance systems and wavelength multiplex transmissions.
In JP-A No. 11-119045, there is also proposed a dispersion shifted fiber that suppresses the enlargement of the Aeff and gives priority to reducing the dispersion slope.
The dispersion slope shows the wavelength dependency of the chromatic dispersion values and is a gradient of the curve when the chromatic dispersion values are plotted when the horizontal axis is set as the wavelength and the vertical axis is set as the chromatic dispersion values. In wavelength multiplex transmissions, the larger the dispersion slope of the transmission path, the larger the difference in the chromatic dispersion values between each wavelength, the more irregular the transmission state, and the more the overall transmission characteristics are deteriorated.
Further, what is known as NZDSF (non zero dispersion shifted fiber) has also been proposed. In NZDSF, because it is easy for four-wave mixing, which is one of the non-linear effects, to occur if the chromatic dispersion value is zero, chromatic dispersion values whose absolute value, although small, is not zero are set.
FIGS. 5A to 5C show examples of the refractive index distribution configuration (i.e. the refractive index profile) used in NZDSF and in the conventional proposed dispersion shifted fibers.
FIG. 5A shows an example of a dual shape core type (step type) of refractive index profile. A core 4 is formed provided with a central core portion 1 and a step core portion 2 provided at the outer periphery of the central core portion 1 and having a lower refractive index than the central core portion 1. In addition, cladding 7 having a lower refractive index than the step core portion 2 is provided at the outer periphery of the core 4.
FIG. 5B shows an example of a segment core type of refractive index profile. A core 24 is formed provided with a central core portion 21 having a high refractive index and an intermediate portion 22 having a low refractive index at the outer periphery of the central core portion 21. A ring core portion 23 having a lower refractive index than the central core portion 21 and a higher refractive index than the intermediate portion 22 is further provided at the outer periphery of the intermediate portion 22. In addition, cladding 27 provided with a first cladding 25 having a lower refractive index than the intermediate portion 22 and a second cladding 26 having a higher refractive index than the first cladding 25 and a lower refractive index than the intermediate portion 22 is formed at the outer periphery of the ring core portion 23.
FIG. 5C shows an example of an O ring type (i.e. a convex type) of refractive index profile. A core 34 having a two layer structure is formed with a central core portion 31 having a low refractive index in the center thereof and a peripheral core portion 32 having a high refractive index provided at the outer periphery of the central core portion 31. A three layer (including the cladding 37) structure refractive index profile is formed by providing cladding 37 having a lower refractive index than the peripheral core portion 32 at the outer periphery of the core 34.
Conventional dispersion shifted fibers and the like that have these refractive index profiles are advantageous with regard to the design of the system, in view of the transmission speed and accumulated dispersion (the chromatic dispersion accumulated by the transmission) when transmitting over long distances, because the chromatic dispersion values in the used wavelength band (operating wavelength band) are small.
When a chromatic dispersion value is set as a negative value, then it is possible to build a system that comparatively easily compensates the chromatic dispersion value in combination with a typical 1.3 xcexcm single mode optical fiber (1.3 SMF).
Namely, a 1.3 xcexcm single mode optical fiber has a zero dispersion wavelength (i.e. when the chromatic dispersion value is zero) of approximately 1.3 xcexcm and hitherto has been well used. Moreover, it has a comparatively large positive value (for example, slightly less than 17 ps/km/nm) as the chromatic dispersion value in the 1.55 xcexcm band. Therefore, it is possible to reduce the chromatic dispersion of the overall system by connecting a 1.3 xcexcm single mode optical fiber to the output side of a dispersion shifted fiber having a negative chromatic dispersion value, and by compensating the negative chromatic dispersion accumulated due to transmission through the dispersion shifted fiber with the positive chromatic dispersion of the 1.3 xcexcm single mode optical fiber.
However, because conventionally proposed dispersion shifted fibers and the like are used as general transmission paths, a small chromatic dispersion is required. For example, there are many cases in which the absolute value of the chromatic dispersion value in the vicinity of 1550 nm is equal to or less than 6 ps/km/nm and if the absolute value of such a chromatic dispersion value is set as a small value, the problem arises that it is difficult to achieve both an enlarged Aeff and a reduction in the dispersion slope.
For example, if an attempt is made to sufficiently enlarge the Aeff, then it is not possible to sufficiently reduce the dispersion slope, and if an attempt is made to sufficiently reduce the dispersion slope, then it is not possible to sufficiently enlarge the Aeff.
Recently, however, as has been disclosed in JP-A No. 6-11620, for example, a system that uses dispersion compensating fiber (abbreviated below to DCF) has been proposed as a different system from one that uses the above type of dispersion shifted fiber having a low chromatic dispersion.
This system uses a fiber for transmission (transmission fiber) that over the majority of the transmission path has a comparatively large chromatic dispersion value in the used wavelength band and is formed by connecting a DCF of a comparatively short length to the output side of the transmission path.
This DCF has a chromatic dispersion value of a different symbol from the chromatic dispersion value of the transmission fiber and a substantially larger value than the absolute value of the chromatic dispersion value of the transmission fiber is selected as the absolute value of this chromatic dispersion value. As a result, it is possible to compensate the chromatic dispersion generated in a transmission fiber over several kilometers or more, for example, using a short DCF at the output side and to thus reduce the chromatic dispersion value of the system as a whole.
Specifically, when the chromatic dispersion value of the transmission fiber is positive, for example, a DCF having a negative chromatic dispersion value with a large absolute value is connected to the output side thereof.
Moreover, what is known as a dispersion slope compensating dispersion compensation fiber (abbreviated below to SCDCF) has been proposed. This SCDCF not only has a chromatic dispersion of a different symbol, but it also has a dispersion slope of a different symbol from the dispersion slope of the transmission fiber, and the chromatic dispersion and the dispersion slope are compensated simultaneously. The SCDCF is used in the same applications as the DCF and is particularly preferable when performing wavelength multiplex transmissions.
In a transmission path in which the SCDCF and the above described transmission fiber are combined, because the local chromatic dispersion values are increased, it is possible to effectively suppress the generation of four-wave mixing, and because substantially flat chromatic dispersion values are obtained over the entire optical communication system, this transmission path is extremely advantageous with regard to transmission loss and is currently being actively developed.
Currently, 1.3 xcexcm single mode optical fiber is generally used as the transmission fiber in systems that use DCF or SCDCF.
FIG. 5D shows the typical refractive index profile of a 1.3 xcexcm single mode optical fiber. A single peak type of refractive index profile is formed from a single layer structure core 44 and single layer structure cladding 47 having a lower refractive index than the core 44 provided at the outer periphery of the core 44.
However, if the typical 1.3 xcexcm single mode optical fiber is used in the 1.55 xcexcm band, although it is possible to obtain an Aeff value of approximately 80 xcexcm2 and a dispersion slope value of approximately 0.06 ps/nm2/km, as described above, the chromatic dispersion value is in the region of 17 ps/km/nm, which is rather large. Therefore, because of the effects from the chromatic dispersion value that accumulate as the optical signal is propagated, the problem has arisen that the transmission distance has been restricted.
Moreover, because the transmission loss is large and the Aeff is small in DCF and SCDCF as compared with typical 1.3 xcexcm single mode optical fiber, there is a large non-linear optical effect. Accordingly, the problem has arisen that the transmission characteristics of the overall system have been deteriorated as the used length has increased.
Further, in the single peak type of refractive index profile used in the 1.3 xcexcm single mode optical fiber, it is possible to make the chromatic dispersion value smaller by adjusting structural parameters such as the core diameter, the relative refractive index difference between the cladding and the core, and the like. However, if the chromatic dispersion value is made smaller within a range in which the bending loss characteristics necessary in the transmission path can be maintained, the Aeff becomes extremely small resulting in the non-linear optical effect becoming too large. This results in it being difficult for the single peak type of refractive index profile used in the 1.3 xcexcm single mode optical fiber to be used in an optical communication system using the aforementioned type of high power signal light.
The present invention was conceived in view of the above circumstances and it is an object thereof to provide a technology that can achieve either one or both of a reduction in cost and an improvement in transmission characteristics in an optical communication system that uses one type or two or more types of optical fiber.
More specifically, it is an object of the present invention to provide a dispersion shifted optical fiber capable of, for example, solving the problem of the difficulty in achieving both a reduction in the dispersion slope and an enlargement of the Aeff of a conventional NZDSF as described above, and utilizing the advantage of four-wave mixing thereof being unlikely to occur of the conventional NZDSF, and also capable of improving the transmission characteristics by suppressing the non-linear optical effect resulting from the enlargement of the Aeff, and of achieving an improvement in the transmission characteristics in wavelength multiplex transmissions by reducing the dispersion slope.
It is a further object of the present invention to provide a dispersion shifted optical fiber that is suitable as a transmission fiber to replace the 1.3 xcexcm single mode optical fiber conventionally used in optical communication systems that use DCF or SCDCF, and in which the absolute value of the chromatic dispersion value is smaller than that of the 1.3 xcexcm single mode optical fiber, and whose chromatic dispersion can be compensated by a short DCF or SCDCF. It is a further object of the present invention to provide a dispersion shifted optical fiber that, as well as being provided with the above characteristics, is capable of suppressing non-linear optical effects of the transmission fiber itself by having a large Aeff, and can be used in wavelength multiplex transmissions as a result of having a small dispersion slope.
It is a further object of the present invention to provide a dispersion shifted optical fiber having as simple a structure as possible and that can be manufactured at a low cost.
In the present invention, a dual shape core type or O ring type of refractive index profile used in NZDSF or in the conventional dispersion shifted optical fiber described above is used. As described above, dispersion shifted optical fibers that have these refractive index profiles were examined principally regarding the setting of the chromatic dispersion value in the 1550 nm band to a value as close to zero as possible.
However, the present inventors realized that the effect of suppressing four-wave mixing obtained by making the chromatic dispersion value larger than in a conventional NZDSF and the phenomenon of being able to achieve both an enlargement of the Aeff and a reduction in the dispersion slope would be effective in wavelength multiplex systems. They also realized that if an optical fiber whose chromatic dispersion value was set smaller than that of a 1.3 xcexcm single mode optical fiber used in a 1.55 xcexcm band wavelength multiplex system in combination with DCF or SCDCF could be achieved, then it would be possible to put together a system suitable for even faster transmissions over longer distances. Moreover, in this type of optical fiber, if the Aeff could be enlarged more than for a 1.3 xcexcm single mode optical fiber, then it would be possible to reduce the non-linear optical effect more than when the 1.3 xcexcm single mode optical fiber is used.
Therefore, the present inventors conducted investigations with the specific object of obtaining a small dispersion slope and large Aeff that had not hitherto been obtainable in a conventional optical fiber for wavelength multiplex transmissions by setting the chromatic dispersion value larger compared to a conventional NZDSF and smaller than a 1.3 xcexcm single mode optical fiber.
The present inventors also discovered that, in the above described refractive index profile, it was possible to obtain one of 1) and 2) below by setting the chromatic dispersion value to between 7 and 15 ps/km/nm.
1) An NZDSF able to achieve both a reduction in the dispersion slope and an enlargement in the Aeff.
2) A dispersion shifted optical fiber with a larger Aeff than in a typical 1.3 xcexcm single mode optical fiber.
In the case of 1), it is possible to achieve both a reduction in the dispersion slope and an enlargement in the Aeff that is not possible with a conventional NZDSF. Moreover, in this dispersion shifted optical fiber, the zero dispersion wavelength moves to a shorter wavelength than 1490 nm. Therefore, wavelength multiplex transmission becomes possible not only in the C-band and the L-band but also in the S-band, and an effect of a size that was not possible with a conventional NZDSF was able to be obtained.
In the case of 2), the obtained fiber is particularly effective as a transmission path used in combination with DCF or SCDCF. Moreover, because it has smaller chromatic dispersion values than a 1.3 xcexcm single mode optical fiber, it is effective in a high speed transmission system. Because it has a large Aeff, it is also possible to reduce the non-linear effect, and is therefore effective in ultra long distance transmission systems such as undersea systems.
Either of the above cases, it is possible to set the dispersion slope to 0.09 ps/km/nm2 or less and to 0.07 ps/km/nm2 or less depending on the design. Accordingly, the unevenness in the chromatic dispersion values is small relative to the wavelength and this is ideal in a wavelength multiplex transmission system.
Specifically, the means for solving these problems described below is proposed.
Namely, the first aspect of the present invention is a dispersion shifted optical fiber, wherein having, in a used wavelength band that is selected from between 1490 and 1625 nm, chromatic dispersion values of 7 to 15 ps/km/nm, an Aeff of 60 to 150 xcexcm2, a dispersion slope of 0.09 ps/km/nm2 or less, a bending loss of 100 dB/m or less, and a cutoff wavelength that provides essentially single mode transmission.
The second aspect of the present invention is the dispersion shifted optical fiber according to the first aspect, wherein the dispersion shifted optical fiber has a refractive index profile comprising: a central core portion; a step core portion provided at an outer periphery of the central core portion and having a refractive index lower than that of the central core portion; and cladding provided at an outer periphery of the step core portion and having a refractive index lower than that of the step core portion.
The third aspect of the present invention is the dispersion shifted optical fiber according to the second aspect, wherein having an Aeff of 60 to 110 xcexcm2 and a dispersion slope of 0.08 ps/km/nm2 or less.
The fourth aspect of the present invention is the dispersion shifted optical fiber according to the third aspect, wherein if xcex941 is a relative refractive index difference of the central core portion and xcex942 is a relative refractive index difference of the step core portion when the cladding is taken as a reference, r1 is a radius of the central core portion, and r2 is a radius of the step core portion, then:
xcex941 is 0.25 to 0.55%,
r2/r1 is 1.5 to 5.0, and
xcex942/xcex941 is between 0.025 or more and a value determined by xe2x88x920.06xc3x97(r2/r1)+0.5 or less.
The fifth aspect of the present invention is the dispersion shifted optical fiber according to the third aspect, wherein having chromatic dispersion values of 7 to 11 ps/km/nm, an Aeff of 60 to 80 xcexcm2, and a dispersion slope of 0.07 ps/km/nm2 or less.
The sixth aspect of the present invention is the dispersion shifted optical fiber according to the fifth aspect, wherein if xcex941 is a relative refractive index difference of the central core portion and xcex942 is a relative refractive index difference of the step core portion when the cladding is taken as a reference, r1 is a radius of the central core portion, and r2 is a radius of the step core portion, then:
xcex941 is 0.4 to 0.5%,
r2/r1 is 3.5 to 5.0, and
xcex942/xcex941 is between 0.025 or more and a value determined by xe2x88x920.06xc3x97(r2/r1)+0.5 or less.
The seventh aspect of the present invention is the dispersion shifted optical fiber according to the third aspect, wherein having chromatic dispersion values of 12 to 15 ps/km/nm, an Aeff of 90 to 110 xcexcm2, and a dispersion slope of 0.08 ps/km/nm2 or less.
The eighth aspect of the present invention is the dispersion shifted optical fiber according to the seventh aspect, wherein if xcex941 is a relative refractive index difference of the central core portion and xcex942 is a relative refractive index difference of the step core portion when the cladding is taken as a reference, r1 is a radius of the central core portion, and r2 is a radius of the step core portion, then:
xcex941 is 0.4 to 0.5%,
r2/r1 is 2.0 to 4.0, and
xcex942/xcex941 is between 0.025 or more and a value determined by xe2x88x920.06xc3x97(r2/r1)+0.5 or less.
The ninth aspect of the present invention is the dispersion shifted optical fiber according to the first aspect, wherein the dispersion shifted optical fiber has a refractive index profile comprising: a central core portion; a peripheral core portion provided at an outer periphery of the central core portion and having a refractive index higher than that of the central core portion; and cladding provided at an outer periphery of the peripheral core portion and having a refractive index lower than that of the peripheral core portion.
The tenth aspect of the present invention is the dispersion shifted optical fiber according to the ninth aspect, wherein if xcex9411 is a relative refractive index difference of the central core portion and xcex9412 is a relative refractive index difference of the peripheral core portion when the cladding is taken as a reference, r11 is a radius of the central core portion, and r12 is a radius of the peripheral core portion, then:
1.3xe2x89xa6r12/r11xe2x89xa62.5,
xcex9411xe2x89xa60.3%,
xcex9412xe2x89xa70.5%,
(xcex9412xe2x88x92xcex9411)xe2x89xa61.2%, and
0.9xe2x89xa6xcex9412xc3x97r12/r11xe2x89xa61.7.
The eleventh aspect of the present invention is the dispersion shifted optical fiber according to the ninth aspect, wherein having an Aeff of 70 to 100 xcexcm2 and a dispersion slope of 0.07 ps/km/nm2 or less.
The twelfth aspect of the present invention is the dispersion shifted optical fiber according to the eleventh aspect, wherein if xcex9411 is a relative refractive index difference of the central core portion and xcex9412 is a relative refractive index difference of the peripheral core portion when the cladding is taken as a reference, r11 is a radius of the central core portion, and r12 is a radius of the peripheral core portion, then when
1.3xe2x89xa6r12/r11xe2x89xa6 less than 2.5,
xcex9411xe2x89xa60.3%,
xcex9412xe2x89xa70.5%,
(xcex9412xe2x88x92xcex9411)xe2x89xa61.2%,
0.9xe2x89xa6xcex9412xc3x97r12/r11xe2x89xa61.7, and
xcex9411=axc3x97xcex9412+b, then
a is represented by the r12/r11 function cxc3x97(r12/r11xe2x88x921)
c is between 1.5 and 2.0
b is represented by the r12/r11 function 0.4xc3x97(r12/r11)+e
e is between 0 and 0.4.
The thirteenth aspect of the present invention is the dispersion shifted optical fiber according to the ninth aspect, wherein having an Aeff of 90 to 150 xcexcm2 and a dispersion slope of 0.08 ps/km/nm2 or less.
The fourteenth aspect of the present invention is the dispersion shifted optical fiber according to the thirteenth aspect, wherein if xcex9411 is a relative refractive index difference of the central core portion and xcex9412 is a relative refractive index difference of the peripheral core portion when the cladding is taken as a reference, r11 is a radius of the central core portion, and r12 is a radius of the peripheral core portion, then:
1.3xe2x89xa6r12/r11xe2x89xa62.5,
xcex9411xe2x89xa60.15%,
xcex9412xe2x89xa70.5%,
(xcex9412xe2x88x92xcex9411)xe2x89xa61.2%, and
1.0xe2x89xa6xcex9412xc3x97r12/r11xe2x89xa61.5.
The fifteenth aspect of the present invention is an optical communication system wherein using a combination of:
the dispersion shifted optical fiber according to any of claims 1 to 9, and
a dispersion compensating fiber for compensating a chromatic dispersion of the above dispersion shifted optical fiber or a dispersion slope compensating dispersion compensation optical fiber for compensating a chromatic dispersion and a dispersion slope of the above dispersion shifted optical fiber.